1. Field of the Invention
The present invention generally relates to a mirror assembly, and more particularly it relates to a mirror assembly of the type comprising a mirror made by forming a reflective film on the backside of a transparent plate and a mirror holder having a base plate covering the backside of the mirror and an engaging edge portion engaging with the foreside peripheral portion of the mirror.
2. Description of the related art
Hitherto, a mirror has not generally been used as mirror itself but as a mirror assembly with the mirror combined with a mirror holder of various sizes and configurations suited for the intended use. An example thereof is a mirror assembly using a mirror holder of the aforementioned type and as a mirror in the form of such mirror assembly may be sighted an outside mirror unit such as a door mirror unit for an automobile.
With a mirror assembly of this type, however, it was often the case that glare or glaringness takes place in the peripheral portion of the mirror, this glare interfering with the appearance of the mirror. Hereinafter the glare of the door mirror unit will be explained in detail with reference to FIGS. 5 and 6 showing the essential parts thereof in section.
First, the composition of this door mirror unit will be outlined below. Generally, the door mirror unit is composed of a mirror 3, a mirror holder 5, a mirror case attached to the automobile's body (not shown). The mirror 3 has its peripheral portion held from behind by the mirror holder 5, and the mirror holder is set in the mirror case via a universal joint (not shown) so that the mirror's direction may be set arbitrarily. As a measure to prevent cracking of the mirror 3 when some external force is applied to e.g. the mirror holder, this mirror 3 has chamfers 6 and 8 formed in the fore and back peripheral corners thereof. Since these chamfers 6 and 8 are generally formed by grinding, the chamfered faces look like opaque ground glass. Meanwhile, for the outer peripheral side face 7 of the mirror 3 finishing by grinding is not proper for ensuring the required precision of the outer dimension, and leaving as cut (by diamond cutter) is preferred also from the viewpoint of reducing the number of processing steps. Hence, its peripheral side face has a slight roughness resulting from the cutting. The mirror 3 thus formed is set in the mirror case combined with the mirror holder 5 as mentioned above, but since this is done by first setting the mirror 3 in the heated and expanded mirror holder 5 (since it is normally made of a plastic) and then cooling to shrink the latter, it is impossible to make too large the extension of the engaging edge portion 4, which holds the mirror 3 from behind the foreside peripheral portion thereof, toward the mirror center.
Hence, as shown by an arrow in FIG. 5, rays of light incident upon the peripheral portion of the mirror 3 may possibly reach the outer peripheral side face. In such a case, since the outer peripheral side face 7 is rough as mentioned above, the light ray is reflected irregularly and the irregularly reflected light ray is further reflected by a reflective film 2 toward the driver's side to thereby cause glare. The figure shows the condition in which the incoming light ray at an incident angle of .theta..sub.1 (60.degree. in the figure) is refracted at a refractive angle .theta..sub.2 (35.degree. in the figure) by glass approx. 1.5 in refractive index. This incident angle is quite usual with the relationship between the door mirror unit and the driver's eye taken into account. The range of the incident angle .theta..sub.1 at which the light ray can be incident upon the outer peripheral side face 7 of the mirror 3 actually varies according to the engaging allowance of the engaging edge portion 4 of the mirror holder 5 with respect to the mirror 3, the thickness of the mirror 3 and the like but here the above angle is given as an example.
Meanwhile, the glare in question can be prevented by precluding the irregular reflection of light ray by the outer peripheral side face thus caused, but it is not preferable to finish here by grinding to make it like ground glass as mentioned above from the viewpoint of the number of processing steps. It is, therefore, conceivable that the glare can be prevented by reducing the width of the outer peripheral side face 7 which can possibly cause irregular reflection by somewhat enlarging the chamfer 8 on the backside as shown in FIG. 6 and by letting the refracted light ray being incident upon the ground glass-like chamfer 8. Even in such a case, however, it is essential that the outer peripheral side face 7 be spared to ensure the required precision of the outside dimensions, hence there is a limit for enlarging the chamfer 8 and, as shown by the imaginary line, the glaring reflected light ray from the outer peripheral side face 7 in the figure is re- reflected toward the reflective surface 2 so as to be cast forward with respect to the mirror 3, when the incident angle is .theta..sub.1 ' (75.degree.) and the refractive angle is .theta..sub.2 ' (40.degree.). In the cases as illustrated in FIGS. 5 and 6, too, glare may possibly be caused by progress of light ray in the reverse direction, that is, opposite to the direction indicated by the arrow.